Difference between android runOnUiThread and simple code in java

Assuming that you meant simple code for UIThread code,

What is a thread ?

A thread defines a process running

First runOnUiThread ..

Runs the specified action on the UI thread. If the current thread is the UI thread, then the action is executed immediately. If the current thread is not the UI thread, the action is posted to the event queue of the UI thread.

What is UIThread

• Main thread of execution for your application

• Most of your application code will run here onCreate, onPause, onDestroy, onClick, etc.

  So simply Anything that causes the UI to be updated or changed HAS to happen on the UI thread

When you explicitly spawn a new thread to do work in the background, this code is not run on the UIThread.Now what if you want to do something that changes the UI? Then you are welcome to runOnUiThread

You have to use runOnUiThread() when you want to update your UI from a Non-UI Thread. For eg- If you want to update your UI from a background Thread. You can also use Handler for the same thing.

Normally your code is executed on your UI thread. For longer taking tasks (such as network requests, etc...) you will use a background tasks (Handler, AsyncTask, Thread, ...).

As your Views can only be touched from a UI thread, you use runOnUiThread() if you are executing code in a background thread and you need to update your views, from this background thread.

To explain 'why' Android has the 'runOnUiThread()' option, it is important to understand that java is only used to create the bytecode (dex) that Android uses. The code running on the phone is NOT java.

Additionally, Android threads 'can' have a thing called a 'looper'. This 'looper' is what handles 'tasks(technically runnables and messages)' in order via a queue. The 'main ui thread' has by default a looper already attached to it.

That means that your runnable you created was put onto the looper's queue of the main UI thread. (this is why the runnable is NOT instantaneously ran, but will be ran 'quickly'/'soon')

The reason you'd use a runnable to run code on the UI thread is because you are in some other 'background thread' that you created... and want to update the UI in some way. (Only the UI thread can interact with the UI)